The conventional method of backwashing or cleaning granular media filters, as commonly used for removing suspended solids from waters or wastewaters, is to pass water upward through the bed at sufficiently high velocities that the media are first fluidized, that is, the media grains are suspended or floated in the upward flowing backwash water, and secondly, expanded by as much as 50% over the settled bed height so that particles of entrapped solids can be removed from between the media grains. After a backwash operation of this type, a single-medium filter bed is left stratified so that the smaller grains are in the upper part of the bed and the larger grains are in the lower part of the bed. When multiple-media filters--usually consisting of coal over sand, or coal over sand over garnet--are used, the sizes of the media are selected so that the lower density and usually larger grain size media overlie the higher density and usually smaller grain size media.
When using multiple-media filter beds, a zone of intermixed media may occur in which the small sizes of the higher density media are interspersed with the larger sized grains of the lower density media. The extent of intermixing depends on the relative size and density of the media within each layer and may be controlled to some extent by appropriate selection of size and gradation of each medium and by the rate and duration of backwash.
Conventional filter designs range from those having essentially no intermixing to those having intermixing through most of the bed depth. Filters having as many as four distinct layers distinguished by media type and grain size or both have been used in full-scale water and waste-water filtration plants. The amount of intermixing is, to a large extent, a filter design option.
Conventional fluidization and expansion backwash methods are not always as effective as desired. The ineffectiveness of scouring of solids with water alone is well known, and is the reason for development of auxiliary media scouring techniques such as air wash preceding water wash and media scouring with high velocity jets of water prior to and during water wash. Even these methods are not as effective as desirable and examples of dirty media and "mudballs" remaining after backwashing by such techniques also are well known.
An alternate backwash method is to use air and water simultaneously throughout much of the backwash cycle. Both research tests and field experience have shown this method to be much more effective in cleaning filter media than conventional backwash methods. Prior to the method described by this invention, the simultaneous use of air and water at the same time water is flowing from the backwash collector has been restricted in practice to single-medium beds that did not require stratification or separation into two or more layers of different sizes or types of media. The simultaneous use of air and water for durations longer than required to fill the volume between the lowest water level prevailing at the end of the filtration sequence and the overflow weir of the backwash collector will result in loss of significant amounts of media unless some positive means is used to control media loss.
When simultaneous air-water wash has been used with multiple-media filters, its use has been restricted to a very short period during which water rises from a level slightly above the surface of the media to the edge of the waste backwash water collector. The air flow must then be terminated to prevent media loss, and the water wash is continued, usually at an increased rate, to wash entrapped solids from the bed and to separate the media into respective layers of different size or density. Using conventional media grain-size and density combinations requires that a relatively high rate water-only step follow the air wash or hydraulic scouring to fluidize and expand these media so that solids washout and stratification occur. For example, a dual-media mixture of 1.0 mm effective size (E.S.) coal over 0.5 mm E.S. silica sand can be backwashed using water at a rate as low as 8 gallons per minute per square foot (gpm/sq.ft.) simultaneously with air at rates as low as 2 standard cu.ft. per minute per square foot (scfm/sq/ft.), whereas washout of solids from these two media and subsequent restratification using water alone, as used in conventional backwash methods, require water rates as high as 20 gpm/sq.ft.; and when using larger grain size combinations, even higher rates of water become necessary. However, it is desirable to be able to use a simultaneous air-water wash method with multiple-media filters that can be extended for a longer backwash duration than the rise time from slightly above the surface of the media to the edge of the waste backwash water collector without the loss of media and subsequently will restratify the filter bed in a manner suitable for filtration.